There's been a number of questions regarding disk cloning tools and dd has been suggested at least once. I've already considered using dd myself, mainly because ease of use, and that it's readily available on pretty much all bootable Linux distributions.

What is the best way to use dd for cloning a disk? I did a quick Google search, and the first result was an apparent failed attempt. Is there anything I need to do after using dd, i.e. is there anything that CAN'T be read using dd?

I'm aware how dd works, my question was more in the direction of any known problems related to dd when cloning disks (as described by the link), maybe this wasn't very clear. What his answer contains and yours doesn't is "I've never once had any problems with it". I did upvote your answer too, as you did definitely present some interesting points (I like the one about no progress indication).
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falstroMay 6 '09 at 20:25

It's ironic that Joel linked to the question as a good example of server-fault, although none of the answers were good. There was not one answer among 25 (excluding comments) with the right dd options for skipping bad blocks - which is essential when cloning disks for recovery. I added a better answer, which can clone disks having bad blocks: dd if=/dev/sda of=/dev/sdb bs=4096 conv=sync,noerror
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Sam WatkinsNov 14 '14 at 6:52

26 Answers
26

dd is most certainly the best cloning tool, it will create a 100% replica simply by using the following command. I've never once had any problems with it.

dd if=/dev/sda of=/dev/sdb bs=32M

Be aware that while cloning every byte, you should not use this on a drive or partition that is being used. Especially applications like databases can't cope with this very well and you might end up with corrupted data.

@Eddie - and of course the partition table will be copied too, so if sdb is larger you'll have unused space at the end.
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AlnitakMay 7 '09 at 8:41

88

just be very careful with the 'i' and 'o' letters...
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bandiMay 29 '09 at 21:52

8

Nobody seems to know this trick... dd is an asymmetrical copying program, meaning it will read first, then write, then back. You can pipe dd to itself and force it to perform the copy symmetrically, like this: dd if=/dev/sda | dd of=/dev/sdb. In my tests, running the command without the pipe gave me a throughput of ~112kb/s. With the pipe, I got ~235kb/s. I've never experienced any issues with this method. Good luck!
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MistiryOct 19 '10 at 20:50

Does this also work with "modern" fs such a BTRFS, NILFS, [whatever you can dream of] ?
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Steve SchneppJun 26 '09 at 14:41

DD works on block devices, a level of abstraction lower than the file system, so it should, yes. I haven't actually tried it, though. Hmm, NILFS looks interesting, I'll have to take a look at that.
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David HicksJun 28 '09 at 22:47

Sorry, just checked out NILFS' homepage and realised what you might have meant - can you use DD to copy a snapshot from a NILFS filesystem? I don't know, but it'd be interesting to find out.
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David HicksJun 28 '09 at 23:01

CAUTION: dd'ing a live filesystem can corrupt files. The reason is simple, it has no understanding of the filesystem activity that may be going on, and makes no attempt to mitigate it. If a write is partially underway, you will get a partial write. This is usually not good for things, and generally fatal for databases. Moreover, if you screw up the typo-prone if and of parameters, woe unto you. In most cases, rsync is an equally effective tool written after the advent of multitasking, and will provide consistent views of individual files.

However, DD should accurately capture the bit state of an unmounted drive. Bootloaders, llvm volumes, partition UUIDs and labels, etc. Just make sure that you have a drive capable of mirroring the target drive bit for bit.

You can always use 'sync' to sync the file system to the hdd before running dd.
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LiraNunaJul 17 '09 at 20:08

6

I suspect that sync is not the answer to file corruption problems. What happens if a deamon or something writes more files after the sync, during the dd operation?
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DeletedAug 13 '09 at 10:47

3

It's a good idea to umount the drive first (or remount as read-only) but it's not always possible
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Alex BolotovAug 22 '09 at 9:36

In which case, you use rsync and let it do file handle magic to get a consistent file and let Copy On Write semantics handle the incoming writes.
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jlduggerAug 22 '09 at 21:18

2

I'd like to add that running dd on a mounted filesystem WILL NOT CORRUPT the files on the mounted filesystem, but what is meant here is that the copy of the filesystem will necessarily be in a known good state.
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3moloApr 12 '13 at 10:11

When using dd to clone a disk which may contain bad sectors, use "conv=noerror,sync" to ensure that it doesn't stop when it encounters an error, and fills in the missing sector(s) with null bytes. This is usually the first step I take if trying to recover from a failed or failing disk -- get a copy before doing any recovery attempts, and then do recovery on the good (cloned) disk. I leave it to the recovery tool to cope with any blank sectors that couldn't be copied.

Also, you may find dd's speed can be affected by the bs (block size) setting. I usually try bs=32768, but you might like to test it on your own systems to see what works the fastest for you. (This assumes that you don't need to use a specific block size for another reason, e.g. if you're writing to a tape.)

If you have a disk with bad sectors, you really should be using 'ddrescue' instead of dd. It's much more efficient, and has a much better chance of recovering more data. (Don't get it confused with dd_rescue, which is not as good)
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davrMar 19 '10 at 20:27

should not use a large block size if attempting to skip bad blocks, or it will skip too much. 4096 is large enough.
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Sam WatkinsNov 13 '14 at 2:54

To clone a disk, all you really need to do is specify the input and output to dd:

dd if=/dev/hdb of=/image.img

Of course, make sure that you have proper permissions to read directly from /dev/hdb (I'd recommend running as root), and that /dev/hdb isn't mounted (you don't want to copy while the disk is being changed - mounting as read-only is also acceptable). Once complete, image.img will be a byte-for-byte clone of the entire disk.

There are a few drawbacks to using dd to clone disks. First, dd will copy your entire disk, even empty space, and if done on a large disk can result in an extremely large image file. Second, dd provides absolutely no progress indications, which can be frustrating because the copy takes a long time. Third, if you copy this image to other drives (again, using dd), they must be as large or larger than the original disk, yet you won't be able to use any additional space you may have on the target disk until you resize your partitions.

You can also do a direct disk-to-disk copy:

dd if=/dev/hdb of=/dev/hdc

but you're still subject to the above limitations regarding free space.

As far as issues or gotchas go, dd, for the most part, does an excellent job. However, a while ago I had a hard drive that was about to die, so I used dd to try and copy what information I could off it before it died completely. It was then learned that dd doesn't handle read errors very well - there were several sectors on the disk that dd couldn't read, causing dd to give up and stop the copy. At the time I couldn't find a way to tell dd to continue despite encountering a read error (though it appears as though it does have that setting), so I spent quite a bit of time manually specifying skip and seek to hop over the unreadable sections.

I spent some time researching solutions to this problem (after I had completed the task) and I found a program called ddrescue, which, according to the site, operates like dd but continues reading even if it encounters an error. I've never actually used the program, but it's worth considering, especially if the disk you're copying from is old, which can have bad sectors even if the system appears fine.

You can actually also use a read-only mount. A filesystem can be remounted with: mount -o remount,ro /path/to/device
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Paul de VriezeMay 30 '09 at 15:11

Good point, I added a note in my answer about that.
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Kyle CroninMay 30 '09 at 18:07

I used ddrescue to scrape data off a dying hard drive, and can confirm that it's awesome.
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sleskeJun 23 '09 at 0:45

5

...dd provides absolutely no progress indications... - well this is not true - there is kinda tricky way how to show progress - you have to find out pid of dd process ('ps -a | grep dd') and then send signal USR1 to this process - 'kill -USR1 <dd_pid_here>'(without <>) which force dd to show progress information.
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Michal BernhardJun 19 '11 at 15:51

3

"several sectors on the disk that dd couldn't read": I think that conv=sync,noerror would help.
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GauthierNov 30 '11 at 10:00

If the source drive is damaged at all, you'll have more luck using dd_rhelp with dd_rescue (my personal preference) or GNU ddrescue.

The reason behind this is that, on read errors, dd keeps trying and trying and trying - potentially waiting for a long time for timeouts to occur. dd_rescue does smart things like reading up to an error, then picking a spot further on on the disk and reading backwards to the last error, and dd_rhelp is basically a dd_rescue session manager - cleverly starting and resuming dd_rescue runs to make it quicker again.

The end result of dd_rhelp is maximum data recovered in minimum time. If you leave dd_rhelp running, in the end it does the exact same job as dd in the same time. However, if dd encountered read errors at byte 100 of your 100Gb disk, you'd have to wait a long time to recover the other 9,999,900 bytes*, whereas dd_rhelp+dd_rescue would recover the bulk of the data much faster.

The above copies a compressed image of the source harddrive to a remote system, where it stores it in numbered 2G chunks using the source host's name while keeping you updated on progress.

Note that depending on the size of disk, speed of cpu on source, speed of cpu on destination, speed of network, etc. You may want to skip compression, or do the compression on the remote side, or enable ssh's compression.

This is not quite correct. The 'remote_machine' command is missing something, such as > disk_backup.img or |dd of=/dev/sdb or something else, depending on what you want to do. I'm guessing you don't want to dump a disk image to stdout.
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davrMar 19 '10 at 20:26

1

And throw in gzip on both ends to further minimize the sent data.
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3moloApr 12 '13 at 10:12

To clone a disk, all you really need to do is specify the input and output to dd:

dd if=/dev/hdb of=hdb.img

Of course, make sure that you have proper permissions to read directly from /dev/hdb (I'd recommend running as root), and that /dev/hdb isn't mounted (you don't want to copy while the disk is being changed). Once complete, hdb.img will be a byte-for-byte clone of the entire disk.

There are a few drawbacks to using dd to clone disks. First, dd will copy your entire disk, even empty space, and if done on a large disk can result in an extremely large image file. Second, dd provides absolutely no progress indications, which can be frustrating because the copy takes a long time. Third, if you copy this image to other drives (again, using dd), they must be as large or larger than the original disk, yet you won't be able to use any additional space you may have on the target disk until you resize your partitions.

You can also do a direct disk-to-disk copy:

dd if=/dev/hdb of=/dev/hdc

but you're still subject to the above limitations regarding free space.

The first drawback can be resolved by gzipping the data as you make the copy. For example:

dd if=/dev/hdb | gzip -9 > hdb.img.gz

The second drawback can be resolved by using the pipeview (pv) tool. For example:

You already told the way to overcome the third drawback...resize the partitions. Enlarging a partition is generally a safe and fast operation (versus shrinking or moving, which is slow and more dangerous since it's moving data around).
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davrMar 19 '10 at 20:28

gzipping will not work with a disk that has been used for some time, as it will be filled with either current or deleted data. gzip will only work if the empty space is zero'ed, which is only the case with a brand new disk.
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TozzFeb 22 '12 at 20:51

3

@Tozz: You can improve the compressibility of a filesystem image by filling the filesystem with a file filled with 0's, syncing it to disk, then deleting it. dd if=/dev/zero bs=1M of=/balloon; sync; rm /balloon (Modulo extra intelligence in the filesystem layer.)
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retracileJul 26 '12 at 14:15

welll, just a thouhgt, but couldn't u just use gparted to resive the partition/disk being copied down to whatever is used- then drop dd? Assuming it's a onetime image it should mitigate this issue.
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bbqchickenrobotMar 24 '12 at 19:40

This is kind of a cheap hack, but it's a quick and dirty way to monitor your DD process.

Run your dd command. Open a new shell and do a ps awx to find your dd process' PID. Now in the new shell type watch -n 10 kill -USR1 {pid of your DD process}

This will do nothing in the watch output window, but back in the original DD shell, DD will start outputting status reports every 10 seconds. You can change the -n 10 in the watch command to any other time frame of course.

How to copy using dd (in this case to a remote machine, but the same principle applies to a local copy) which shows progress.

It works by storing the pid via file descriptor 3 in /tmp/pid, which is then used for the subsequent kills with signal USR1. A wrinkle was to filter the output of the progress on stderr to only one line via filtering stderr through a subshell.

Not really relevant to the question, but its a neat shell trick using a sub-shell and high (higher than stderr) file descriptors to convey data out of it, +1
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falstroJul 31 '13 at 6:10

I was referring to this page myself for different dd options when cloning disks, so it seemed a suitable place at the time to put the end result of what I used for cloning, especially since I thought it was rather neat myself :)
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Edward GroenendaalJul 31 '13 at 23:29

I found the documentation a little rough, and cloning a linux PATA drive to a SATA drive did Not leave me with something I could boot (yet). But much faster to same result as dd, and it worked great for my laptop drive upgrades.
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jbdavidMay 30 '09 at 0:43

@Michael the answer you linked does not contain a full example command, and has other faults - it's not a good idea to use large block size with conv=sync,noerror as it will skip too much data for each bad block. These options are essential for "recovery cloning", and it's no good having to search the comments for them. The most popular answer is adequate if the disks have no bad blocks, e.g. for cloning a pristine disk, but not for recovery.
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Sam WatkinsNov 14 '14 at 6:32

@Michael the example I gave is a command I have used several times for professional disk recovery. While there are other tools which might do a slightly better job, the example I gave is better at disk recovery cloning than every other dd example here, while also being suitable for cloning an error-free disk. Therefore, I consider my answer to be the best here on how to "use DD for disk cloning". I did not add info on monitoring progress, compression, etc., because I wanted to keep it simple, and focus on providing a short answer which gets the basics right.
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Sam WatkinsNov 14 '14 at 6:38

disk clusters are generally around 4k now, so using 4096 is probably a good option, and even 8192 if you want to read 2 clusters at a time. Don't go too big though, as you run into fragmented memory problems
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user4767May 29 '09 at 19:32

One thing you must be aware of when dd-ing a full disk is that doing so will overwrite the master boot record of the receiving disk. This contains the partition table and other vital information. If the new disk is not the same as the old disk, this can create all sorts of tables. Copying over partitions is generally safer (and swap partitions do not have to be copied over)

I've been out of the admin role for many years now, but I know that 'dd' is up to the job. I used this technique regularly in the late 80s on Sun Sparc and 386i computers. I had one client order over 30 386i systems running CAD software that was distributed on multiple QIC tapes.

We installed on the first computer, configured the app, ran SunOS' sys-unconfig, placed the drive in a shoebox with a different SCSI address and then proceeded to 'dd' to the other 30 drives.

As others have mentioned above, one of the gotchas with cloning a mounted file system is potential data corruption. This obviously won't apply to full drive clones, but if you're using LVM you can Snapshot the LogicalVolume and dd from the snapshot to get a consistent image.

Just a warning to beginners that needs to be said: At least with some Versions, bs=X means that memory to the size of X will literally be allocated. bs=2GB on a system with 1GB of RAM and insufficient swap WILL cause bad things to happen.